The present invention relates to a screwdriver by an electric motor.
Power screwdrivers are utilized in a field of precision machinery such as assembly of watches.
In screwing with power screwdrivers, the power is increased to strengthen the rotation torque when the driver screw tightens to the point that it cannot rotate anymore. By increasing the power, the screwdriver tries to rotate, and sometimes it rotates a little bit and tightens the screw. The operation is called "MASHIJIME", which means final increased tightening. In "MASHIJIME" the power is gradually increased, which may make the rotation torque of the motor stable and provide a reliable attachment.
Though longer "MASHIJIME" provides more stable torque, in some jobs, shorter assembly time may be needed more than stable torque. Furthermore, "MASHIJIME" time varies depending on the type of screw to be screwed or the production system where the power screwdriver is applied.
The screwdriver occasionally rotates a half revolution or more in the final tightening "MASHIJIME" for several reasons. One reason is that the bit of the driver rotates in the screw head because the bit does not match the head of the screw or because the bit is broken. Such false rotation of the screwdriver causes weak and unreliable fastenings because of the lack of final tightening so that the device so assembled become substandard.
In manual operation, the operator may detect the false rotation of the power screwdriver bit. But today screwdrivers are mostly operated by automatic machines like robots, which makes it difficult to detect false rotation of the screwdriver bit and causes many substandard devices.
An object of the invention is to provide a power screwdriver which can change the final tightening "MASHIJIME" time.
A further object of the invention is to provide a power screwdriver which can detect repetitive occurrence of false rotation of a bit of a screwdriver.